This invention relates to vapor compression distillers.
One process for concentrating liquids for further use or more convenient disposal involves vapor compression distillers wherein the feed solution is subject to evaporation and the residual liquid becomes more concentrated. Such distillers generally include a heat exchange tube bundle over which liquid is sprayed, and through which compressed vapor is passed. A portion of the feed liquid passing over the external surfaces of the heat exchange tube is vaporized and the remaining more concentrated liquid is collected in a well at the lower end of the device while the vapor passing through the tubes is condensed and withdrawn. The vaporized feed liquid is compressed in a vapor compressor and delivered to the interior surfaces of the heat exchange tubes. When the liquid has been concentrated to the desired degree, it may be withdrawn continuously or periodically from the well and fresh feed liquid is added to replenish that which is withdrawn and the portion that is evaporated.
The efficiency of vapor compression distillers is a function of the difference between the temperature at which the liquid boils and the boiling temperature of water at the same pressure which is called the boiling point elevation (BPE) and the viscosity of the liquid. In the treatment of certain liquids, both the boiling point elevation and viscosity rise in relation to the concentration of the liquid. For example, in waste liquids from soft drink bottling plants which contain a sugar concentration of one to two percent, the boiling point elevation is less than one degree and the viscosity is less than one centipoise. However, when such liquids have been concentrated to a thirty percent sugar solution, the boiling point elevation is about 4.degree. F. and the viscosity is three to four centipoise. The liquid being evaporated in conventional vapor compression distillers has substantially the same concentration as the final concentrate. Accordingly, where there is a substantial boiling point elevation and/or viscosity for the concentrate being evaporated, the efficiency of vapor compression distillers is substantially reduced.
When certain solutions are evaporated, such as those containing high concentrations of carbonate ions, large quantities of CO.sub.2 are released. This gas which is non-condensable tends to reduce the temperature at which vapor can exist within the heat exchange tubes. Thus, the temperature differential between the vapor and the water on the outer surface of the heat exchange tubes may become too small for effective evaporation.